Combustion Apparatus

ABSTRACT

A burner for a combustion apparatus, a combustion apparatus comprising such burners, and a method of operation of the same are described. The burner comprises: a fuel input conduit for supplying fuel to the burner; a combustion gas input conduit for supplying combustion gas to a first stage combustion site; an overfire gas input conduit for supplying overfire gas to a second stage combustion site; and gas supply means; wherein the gas supply means is adapted to supply gas switchably between a first mode of operation wherein air is supplied in suitable proportion to both the combustion gas input conduit and the overfire gas input conduit; and a second mode of operation wherein an oxygen containing gas other than air is supplied to the combustion gas input conduit and substantially no gas is supplied to the overfire gas input conduit.

INTRODUCTION

The invention relates to a combustion apparatus, to a fuel burner for acombustion apparatus, and to a method of operating a fuelburner/combustion apparatus. The invention in particular relates to aburner for operation with air or oxyfuel operational conditions.

BACKGROUND

In conventional fossil fuel fired combustion equipment as applied toindustrial and utility boilers for steam generation the oxygen requiredto burn the fuel is supplied via atmospheric air. The combustion of thefuel in air releases the chemical energy stored in the fuel as heat,which is then transferred to the water in the boiler to generate steam.Typically the air is supplied to the boiler via burners, which aretypically of low NOx design, and via overfire air ports. The boiler mayhave one or, more typically, a number of burners and overfire air ports.The burners will typically comprise a fuel stream surrounded by one ormore air streams; the air streams may be swirled, the swirling airproviding the stabilisation of the flame on the burner. The air may besupplied individually to each burner or overfire air port via ducts, orto a group of burners or overfire air ports via a common plenumtypically known as a windbox.

In the burner (or burners), the efficiency of combustion (as indicatedby the level of carbon monoxide and unburned carbon that remains aftercombustion is complete) and propensity for the formation of nitrogenoxides (NOx) are related to the quantity of oxygen provided and the rateof mixing of the oxidising media with the combusting fuel. Since theconcentration of oxygen in air is fixed, it is necessary to adjust theoverall amount of air and/or the proportion of air to the individualregisters, and to adjust the proportion of the air supplied to theburners and the overfire air ports to achieve the optimum systemperformance. However, in a system with fixed burner geometry, the scopefor diverting the air from the burners to the overfire air is limited bythe minimum flow requirement to the burners that is needed to generatesufficient swirling energy to maintain a stable flame.

In one implementation of the oxyfuel technology for fossil fuel firedcombustion equipment, the combustion process will utilise a recycledflue gas stream to which is added upstream of the burners a pure, ornearly pure, oxygen injection stream to produce a single comburant gaswhich gives combustion process performance equivalent to that ofconventional air firing. The oxygen concentration of the comburant gasis variable, and increases with reducing flue gas recycle rate. Thisapproach is sometimes referred to as “simulated air” firing. Aproportion of oxyfuel plant is expected to retain an air firingcapability.

A feature of “simulated air” firing is that the density of the recycledflue gas and oxygen mixture is generally higher than the air which itreplaces and this can lead to reduced local gas velocities within theburner, thereby impairing the stability of a burner designed for airfiring when it is operated in oxyfuel combustion mode. Conversely theburner may be designed for adequate gas velocities under oxyfuel firingconditions, leading to excessive pressure drop across the burner when itis fired with air.

SUMMARY OF INVENTION

According to a first aspect of the present invention, there is provideda burner for a combustion apparatus comprising:

-   -   a fuel input conduit for supplying fuel to the burner;    -   a combustion gas input conduit for supplying combustion gas to a        first stage combustion site;    -   an overfire gas input conduit for supplying overfire gas to a        second stage combustion site; and    -   gas supply means;    -   wherein the gas supply means is adapted to supply gas switchably        between:    -   a first mode of operation wherein air is supplied in suitable        proportion to both the combustion gas input conduit and the        overfire gas input conduit; and    -   a second mode of operation wherein an oxygen containing gas        other than air is supplied to the combustion gas input conduit        and substantially no gas is supplied to the overfire gas input        conduit.

The term “oxygen containing gas other than air” is intended to cover asecond mode gas supply that includes a proportion of oxygen to supportcombustion at the first stage combustion site but that is not simplyair. In particular it is intended to cover a bespoke gas supplycomprising substantially pure oxygen or a mixture of substantially pureoxygen or other oxygen enriched gas (that is, gas with an oxygen contentby volume substantially higher than air) and a second gas, wherein thesecond gas is other than air, and in particular has a reduced nitrogencontent relative to air. Preferably the gas supply means is adapted toselectively supply such a mixture to the combustion gas input conduit.The second gas is preferably recycled flue gas. The second mode gassupply thus preferably comprises oxygen enriched recycled flue gas, andoptionally further component gases. In other words, the second mode thuscomprises an “oxyfuel” firing mode.

The second mode gas supply to the combustion gas input conduit may haveany proportion of oxygen capable of supporting combustion at the firstcombustion site. In a preferred case the second mode gas supply has aproportion of oxygen which generally gives combustion processperformance equivalent to that achieved with air for example beingaround 20 to 50%. That is, the second mode comprises a “simulated air”oxyfuel firing mode.

In accordance with the invention the burner is thus switchable betweentwo modes of operation each having a different gas supply and eachhaving a different combustion process. In the first mode the burner issupplied with air and effects two stage combustion operation, that is,with an appreciable proportion of the total combustion air diverted fromthe burners to overfire air ports. In the second mode the burner issupplied with a bespoke gas comburant and effects a single stagecombustion operation, that is, with notionally all of the oxygencontaining comburant supplied via the burners and notionally nocombustion gas diverted to the overfire air ports.

The invention in a first aspect thus allows the burner to be operatedselectively under air firing conditions or under oxyfuel combustionconditions, in particular “simulated air” oxyfuel combustion conditions,in a manner which optimises the combustion process without the need tochange the burner geometry.

The invention in a first aspect thus comprises a burner that is capableof operation with both air and oxyfuel combustion conditions withpotential reduction in the negative impacts on flame stability, turndownperformance and pressure drop arising from the higher density of fluegas compared to air.

Preferably the gas supply means is adapted to switchably supply eitherair or the oxygen containing gas other than air additionally to the fuelinput conduit such that the gas at least assists in transporting thefuel to the combustion chamber. Thus the gas supply means is adapted toswitchably supply either air or the oxygen containing gas other than airto both of the fuel input conduit and the combustion gas input conduit.Preferably the gas supply means includes varying means, such as a baffleor valve means, for varying the proportion of gas supplied to one orboth or all where applicable of the fuel input conduit and thecombustion gas input conduit and the overfire gas input conduit.

A burner thus comprises a primary fuel input conduit supplying fuel tothe burner, a secondary combustion gas input conduit supplying comburantgas to a first stage combustion site, and a selectively operableoverfire gas input conduit supplying comburant gas to a second stagecombustion site. The combustion gas input conduit may comprise pluralfluidly independent input conduits, for example comprising additionaltertiary or higher order input conduits, fluidly connected to the gassupply means for supplying air or oxygen containing gas to a combustionsite defined by the burner, for example directly to the burner orotherwise to a combustion apparatus in which the burner is located. In atypical arrangement, a primary fuel input conduit may extend along aburner, a secondary combustion gas input conduit may be disposedoutwardly of and for example annularly arrayed about the primary fuelinput conduit, and higher order combustion gas input conduits, wherepresent, may be disposed outwardly of and for example annularly arrayedabout the secondary input conduit in familiar manner. The primary inputconduit may be a central conduit extending generally axially along theburner, for example on a burner centreline. Alternatively, the primaryinput conduit may itself be disposed about a central conduit, forexample annularly, with the central conduit serving another purpose. Theprimary input conduit is still preferably nearer to the centre line thanthe secondary and higher order conduits, but the primary stream does notnecessarily flow along the centreline itself. A conduit may includesuitable swirl generation structures to impart an axial swirl to a gassupply therein.

In a more complete aspect of the present invention, there is provided acombustion apparatus comprising:

-   -   a combustion chamber; and    -   at least one and preferably a plurality of burners as        hereinbefore described located so as to define combustion sites        within the combustion chamber.

Preferably the combustion apparatus comprises a boiler for generatingsteam.

Preferably the fuel used is coal, most preferably pulverised coal.

Preferably the combustion apparatus includes a flue gas recirculationconduit. Preferably the flue gas recirculation conduit is fluidlyconnected to the gas supply means and/ or to at least one of the fuelinput conduit and the combustion gas input conduit such that a mixtureincluding flue gas may be supplied to the combustion chamber during thesecond mode of operation. Preferably the flue gas recirculation conduitis fluidly connected to both of the fuel input conduit and thecombustion gas input conduit.

According to a further aspect of the present invention, there isprovided a method of firing a burner in a combustion apparatusselectively in two modes of operation comprising in a first mode ofoperation:

-   -   supplying fuel to the burner;    -   supplying combustion gas comprising air to a first stage        combustion site;    -   supplying overfire gas comprising air to a second stage        combustion site;    -   causing combustion of fuel to occur in a two stage process; and        in a second mode of operation:    -   supplying fuel to the burner;    -   supplying combustion gas comprising an oxygen containing gas        other than air to a first stage combustion site;    -   causing combustion of fuel to occur in a single stage process.

Preferably the oxygen containing gas comprises a mixture ofsubstantially pure oxygen and a second gas having a reduced oxygenconcentration relative to air and/or a reduced nitrogen concentrationrelative to air. In a preferred embodiment of the second mode,combustion gases are supplied to the combustion site in such manner thatthe resultant mixture has a proportion of oxygen that producescombustion process performance equivalent to that achieved with air, forexample being around 20 to 50%.

Other preferred aspects of the method, and in particular modes ofoperation and gas compositions, will be appreciated by analogy.

SUMMARY OF FIGURES

The invention is described by way of example only with reference toFIGS. 1 to 3 of the accompanying drawings in which:

FIG. 1 is a schematic of a burner suitable for both conventional airfiring and oxyfuel firing in accordance with the invention;

FIG. 2 is a schematic of the overfire principle;

FIG. 3 is a schematic of the oxyfuel process.

SPECIFIC DESCRIPTION

FIG. 1 is a schematic of a simple burner suitable for both air firingand oxyfuel firing in accordance with the invention.

In an air fired mode the combustion air, containing the required oxygento burn the fuel, is supplied by means of a forced draught fan (FD fan)to individual burners via air ducts, or to groups of burners through awindbox. In the example shown, there are three separate air streams inthe burner (1); the primary air (PA) which conveys the coal, thesecondary air (SA), and the tertiary air (TA); specific burner designsmay have fewer or more air streams. In the example shown, the primaryair (PA) stream follows the burner axis (11) and the secondary air (SA),and the tertiary air (TA) streams are axially directed in ductsconcentrically therearound. The burner (1) is fired through an outlet ina furnace wall (FW).

In the embodiment the primary air stream is in a central conduit. Theinvention is not limited to such arrangements. One alternative designoption involves moving the primary air (or PFGR/Oxygen/Fuel mix) to aconduit annularly parallel to the central line, but with a centralcylinder along the axis used for other purposes (core air and/or foroil/gas igniters). In this case, the primary stream would still benearer the centre line than the secondary and tertiary streams, butwouldn't flow along the actual centerline.

Dampers (3) control the division of the air between the secondary andtertiary streams (SA, TA). The secondary and tertiary air streams may beswirled, and the extent of swirl may be adjustable. Swirl devices (5)are provided downstream of the dampers for this purpose. Optimisation ofthe burner with respect to combustion efficiency, emissions, and flamestability is achieved by variation of the total quantity of the airsupplied to the burner, the division of the air between the variousstreams, and the level of swirl applied.

For NOx control a proportion of the total combustion air may be divertedaway from the burners as overfire air, and the flow area of thesecondary and tertiary annuli may be reduced in proportion so as tomaintain the air velocities within the burner registers, and hencemaintain the flame stabilisation effect induced by the swirl generation.FIG. 2 is a general schematic of the overfire principle, with overfireports located generally above the burners which may be of the generalform illustrated in FIG. 1.

The figure shows a typical coal-fired power station arrangement. Acombustion chamber (13) is supplied with coal and combustion air via amultiplicity of burners (15). The air supply to each burner is splitinto primary air (to convey the coal), and secondary and tertiary air tothe windbox (16) (the split being to control the mixing and aerodynamicsin a low NOx burner). In modern plant a proportion of the combustion airis supplied separately as overfire air through a multiplicity of ports(17) (generally above the burners) to facilitate NOx control.

FIG. 3 is a schematic of the oxyfuel process.

In the “simulated air” exemplification of oxyfuel firing shown here,flue gas is recycled to the coal pulverising mill (21) (primary flue gasrecycle, or PFGR) and to the windbox (23) containing the burners (notspecifically shown in FIG. 3) (secondary flue gas recycle, or SFGR) bymeans of dedicated fans (respectively the primary flue gas recycle fan25 and the secondary flue gas recycle fan 27); there are numerousdetailed variants of this process, but they all follow the same genericmethod.

The composition of the recycled flue gas is related to the combustionprocess, but the stream extracted from the boiler exit will contain lowlevels of oxygen, typically less than 5% by volume, and insufficient tosupport combustion. Pure, or nearly pure, oxygen is introduced into thePFGR and SFGR streams to provide the oxidant required to combust thefuel. The composition of the PFGR and SFGR streams will depend upon thedetailed implementation of the oxyfuel technology, but typically thePFGR will contain around 20 to 25% by volume of oxygen or higher,whereas the SFGR will contain a significantly higher oxygenconcentration, for example 25 to 50% . The exact concentration levelswill be dependent upon a number of factors including the overall furnacestoichiometry, the quantity of flue gas that is recycled to the boiler,the amount of combustion generated moisture that is removed from therecycle stream, the amount of air that leaks into the process, etc.

The streams supply burners (not specifically shown) via the windbox (23)to fire the furnace/boiler (31) in generally known manner, with fluegases being drawn off via a particulate removal system (33) to removesolids (ash, etc, 34) and drawn by means of an induced draught (ID) fan(35) to a stack or capture stage (37) as will be familiar.

A key process design parameter for the “simulated air” implementation ofoxyfuel technology is the quantity of recycled flue gas, usuallyexpressed as a percentage of the total flow in the boiler. Thisparameter is selected on the basis of maintaining, as far as ispossible, the radiative and convective heat transfer properties of anoxyfuel fired boiler relative to air firing; recycle gas flow rates ofbetween 60% and 80% of the boiler gas flow are often quoted. Based uponthese typical quantities, and the physical properties of theoxygen/recycled flue gas mixture, it is possible to establish thevelocities within the burner when it is operated under “simulated air”oxyfuel firing conditions. As noted previously, gas velocities withinthe burner are lower for oxyfuel firing than for air firing.

Consider by way of illustration a “Burner A” comprising a simpleconventional air-fire operation, and a “Burner B” comprising a stagefired operation switchable in accordance with the present invention.Example operational data are set out in the table below.

Non-Dimensionalised Burner A Burner B Air Oxyfuel Air Oxyfuel BurnerStoichiometry (—) 1.17 1.17 1.00 1.17 Mass Flow (%) Primary 19 22 19 22Secondary + Core 15 12 13 12 Tertiary 66 66 53 66 Overfire 0 0 15 0Total 100 100 100 100 Velocity Primary base 7.9 base 7.9 (% age change)Tertiary base −20.7 base −2.4

Consider “Burner A”. This represents a typical unstaged low NOx burner.The primary air velocity is selected to ensure the effective conveyingof the transported coal dust to the burner exit, while the tertiary airvelocity is selected to ensure sufficient forward momentum through theswirl generators to achieve the required aerodynamics.

In moving to oxyfuel combustion the air is replaced by recycled flue gas(FGR), comprising predominantly CO₂ and H₂O. The density of the FGR isgreater than that of the air which it replaces. Thus there are a numberof consequences arising from the operation of “Burner A” under oxyfuelconditions:

-   -   1) The mass of primary FGR is greater than the mass of primary        air so that the primary velocity is maintained at a similar        value to the air firing case    -   2) The mass of tertiary FGR is correspondingly reduced from the        value it would otherwise have been    -   3) Furthermore the density of the tertiary FGR is greater than        air    -   4) The velocity of the tertiary FGR is significantly lower than        that of the tertiary air    -   5) The reduced mass flow and velocity leads to lower tertiary        FGR momentum compared to air firing, with an adverse impact on        flame aerodynamics

As a result, “Burner A” cannot be operated in both air and oxyfuelfiring modes without modification.

Now consider “Burner B”. This is an air fired burner designed to operateunder air staging conditions (where some of the combustion air isdiverted away from the burners to overfire air ports). The primary airvelocity is maintained to ensure the effective conveying of the coal tothe burner outlet as previously, and the burner diameter is reduced sothat the tertiary air velocity is maintained at the previous value inspite of the reduced mass flow.

Operating “Burner B” under unstaged oxyfuel firing at the sameconditions as defined previously for “Burner A” demonstrates that, byretaining all the FGR flow through the burner, both the primary andtertiary FGR velocities can be maintained at the same value as thestaged air combustion operating conditions. Thus “Burner B” can beoperated in both air firing and oxyfuel firing modes by provision merelyof suitable means to switch supply conditions between the two modes andwithout any substantive modification to burner geometry.

The invention thus comprises a design of burner that can operate undereither air firing or oxyfuel (“simulated air”) firing without the needfor geometric or other process changes. The burner is sized to operateunder air staging conditions with a stoichiometric air to fuel ratiolower than that typical of single stage operation. When operating thisburner with the increased mass flowrate for single stage oxyfuelcombustion the gas velocities within the burner approach those forconventional staged air firing, and the burner's performance with regardto both flame stability, load reduction, and pressure drop isunimpaired.

The invention allows the burner to be operated under air firingconditions. The invention allows the same burner to be operated under“simulated air” oxyfuel combustion conditions.

The invention allows both of the above without the need to change theburner geometry, settings, or relative flow splits between secondary andtertiary (or other) registers.

The invention allows the burner to operate in air or oxyfuel mode withacceptable velocities for the maintenance of both flame stability andacceptable pressure drop across the full normal operational load rangeof 40% to 110% of the normal rated burner capacity.

1. A burner for a combustion apparatus comprising: a. a fuel inputconduit for supplying fuel to the burner; b. a combustion gas inputconduit for supplying combustion gas to a first stage combustion site;c. an overfire gas input conduit for supplying overfire gas to a secondstage combustion site; and d. gas supply means; wherein the gas supplymeans is adapted to supply gas switchably between: a first mode ofoperation wherein air is supplied in suitable proportion to both thecombustion gas input conduit and the overfire gas input conduit; and asecond mode of operation wherein an oxygen containing gas other than airis supplied to the combustion gas input conduit and: substantially nogas is supplied to the overfire gas input conduit.
 2. A burner inaccordance with claim 1 wherein the gas supply means is adapted tosupply an oxygen containing gas other than air comprising a mixture ofan oxygen enriched gas and a second gas.
 3. A burner in accordance withclaim 2 wherein the oxygen enriched gas is substantially pure oxygen. 4.A burner in accordance with claim 2 wherein the second gas has a reducednitrogen, content relative to air.
 5. A burner in accordance with claim4 wherein the second gas is recycled flue gas.
 6. A burner in accordancewith claim 1 wherein the oxygen containing gas other than air has aproportion of oxygen which gives a, combustion process performancegenerally equivalent to that achieved with air, for example being around20 to 50% by volume.
 7. A burner in accordance with claim 1 wherein thegas supply means is adapted to switchably supply either air or theoxygen containing gas other than air additionally to the fuel inputconduit to at least assist in transporting the fuel to the combustionchamber.
 8. A burner in accordance with claim 7 wherein the gas supplymeans includes varying means for varying the proportion of gas suppliedto one or more of the fuel input conduit, the combustion gas inputconduit and the overfire gas input conduit.
 9. A burner in accordancewith claim 1 wherein the combustion gas input conduit comprises pluralfluidly independent input conduits fluidly connected to the gas supplymeans.
 10. A combustion-apparatus comprising: a. a combustion chamber;and b. one or more burners located so as to define combustion siteswithin the combustion chamber, each burner comprising a fuel inputconduit for supplying fuel to the burner, a combustion gas input conduitfor supplying combustion gas to a first stage combustion site, anoverfire gas input conduit for supplying overfire gas to a second stagecombustion site, and gas supply means, wherein the gas supply means isadapted to supply gas switchably between a first mode of, operationwherein air is supplied in suitable proportion to both the combustiongas input conduit and the overfire gas input conduit, and a second modeof operation wherein an oxygen containing gas other than air is suppliedto the combustion gas input conduit and substantially no gas is suppliedto the overfire input conduit.
 11. A combustion apparatus in accordancewith claim 10 comprising a boiler for generating steam.
 12. A combustionapparatus in accordance with claim 10 comprising a fuel supply ofpulverized coal.
 13. A combustion apparatus in accordance with claim 10wherein the combustion gas input conduit is a first combustion gas inputconduit comprising a part of the burner and including at least onefurther fluidly independent combustion gas input conduit fluidlyconnected to the gas supply means for supplying gas directly to at leastone of the burner and another component of the combustion apparatus. 14.A combustion apparatus in accordance with claim 10 including a flue gasrecirculation conduit.
 15. A combustion apparatus in accordance withclaim 14 wherein the flue gas recirculation conduit is fluidly connectedto one or more of the gas supply means and at least one of the fuelinput conduit and the combustion gas input conduit such that a mixture,including flue gas may be supplied to the combustion chamber in use inthe, second mode of operation.
 16. A method of firing a burner in acombustion apparatus comprising firing the burner selectively in one oftwo modes of operation, comprising in a first mode of operation:supplying fuel to the burner; supplying combustion gas comprising air toa first stage combustion site; supplying overfire gas comprising air toa second stage combustion site; causing combustion of fuel to occur in atwo stage process; and in a second mode of operation: supplying fuel tothe burner; supplying combustion gas comprising an oxygen containing gasother than air to a first stage combustion site; causing combustion offuel to occur in a single stage process.